The present invention refers in general to a control device for a gearbox, in particular for a motor-vehicle gearbox, comprising a control lever operable by the driver and at least one elongated mechanical transmission member interposed between the gearbox and the control lever to transmit the engagement commands imparted by the driver through the control lever to at least one engagement member within the gearbox, and more particularly to a system for identifying the engaged gear to be installed in a gearbox control device to provide a signal indicative of the gear currently engaged.
The term “elongated mechanical transmission member” used above is to be intended as referred, in the following description and claims, to any mechanical member able to transmit a command by means of a translational movement. Such a mechanical member may be either a rigid member, typically a rod, or a flexible member, typically a Bowden cable.
U.S. Pat. No. 6,569,45 discloses a control device for a bicycle gearbox including a flexible control cable for transmitting the commands imparted by the rider through a shift lever mounted on the handlebar to a (front or rear) derailleur of the gearbox, as well as a system for identifying the engaged gear, which system comprises a movable member directly secured to the control cable and provided with a movable contact, a tubular housing mounted between two sheath sections within which the control cable is slidable and conductive tracks provided on the surface of a cavity of the housing along which tracks the movable contact is slidable, in such a manner that the linear position of the control cable, and hence the engaged gear, can be assessed by measuring a change in the electrical resistance resulting from the displacement of the movable contact along the tracks. However, such a system for identifying the engaged gear has the drawback that it requires to mount the movable member directly on the control cable and can therefore be used only in a gearbox control device provided with a flexible, push-pull cable, i.e. a flexible cable which is guided both in the pulling direction and in the pushing direction. Therefore, it cannot be installed at the ends (either on the side of the control lever or on the side of the gearbox) of the control cables of the usual control devices for motor-vehicle gearboxes, as these ends are formed by rigid rod-like portions which have to be suitably guided and supported and thus cannot have movable members directly mounted thereon.
EP-A-0 820 926 discloses a control device for a bicycle gearbox including a control cable arranged to transmit to a rear derailleur of the gearbox the commands imparted by the rider through a shift knob mounted on the handlebar and a system for identifying the engaged gear, which system comprises a movable member provided with a magnet directly mounted on the control cable and a contactless magnetic position sensor (including for example a plurality of Hall sensors) arranged to detect the position of the movable member. In addition to the drawback that it cannot be installed on the rigid end portion of a control cable of a gear shift control device of a motor vehicle due to the same reasons as those explained above, the system for identifying the engaged gear which is known from the above-mentioned European Patent application has the drawback that the mounting of a magnetic movable element on a steel control cable involves the magnetization of the cable and hence a loss in the sensitivity of the sensor.
EP-A1 154 174 discloses a control device for a motor-vehicle gearbox comprising a control lever operable by the driver, a servomechanism for assisting the engagement commands imparted by the driver by means of the control lever and a cable connecting the control lever with the servomechanism. The servomechanism comprises an actuating rod, a gear motor for driving the translational movement of the actuating rod under the control of an electronic control unit, a control rod mounted so as to be able to translate parallel to the actuating rod, a connecting lever which is articulated at an intermediate point thereof by means of an articulation pin to the actuating rod, is connected at a first end thereof to the cable and is connected at the opposite end thereof by means of a universal joint to an end of the control rod, and a displacement sensor associated to the end of the control rod opposite to the end connected power rod. When the driver operates the control lever, the electronic control unit controls the motor to generate an assisting force on the actuating rod on the base of the signal provided by the displacement sensor as a result of the displacement of the control rod which is connected to the cable through the connecting lever. Since the connecting lever is not stationary, but moves itself as a result of the movement imparted to the actuating rod by the gear motor, the displacement of the control rod detected by the displacement sensor does not correspond, i.e. is not univocally linked, to the displacement of the cable. The control device therefore requires an additional sensor arranged to provide the electronic control unit with a signal indicative of the engagement position of the gearbox.